Local structure of Cu²⁺ in Cu-doped hexagonal turbostratic birnessite and Cu²⁺ stability under acid treatment

Geochemical behaviors of heavy metal contaminants, such as Cu2+, are strongly controlled by natural birnessite-like minerals in both marine and terrestrial environments. However, the mechanisms of the interaction of Cu2+ with birnessite are not fully understood yet. In the present study, Cu2+ was coprecipitated with Mn2+ to produce hexagonal turbostratic birnessite, which is analogous to natural birnessite. The obtained Cu-doped birnessite was characterized by powder X-ray diffraction, field-emission scanning electron microscopy, and X-ray absorption spectroscopy (XANES+EXAFS). The stability of Cu(II) in the birnessite structure was investigated by acid treatment. Increasing the dopant content reduces the mineral crystallinity in the [001] direction and the unit cell parameter b from the hexagonal layers. It also shortens the bond length of MnO in the [MnO6] unit and the edge-sharing MnMn distance in the layers, and increases the average oxidation state (AOS) of Mn and the specific surface area. Analysis of Cu K-edge XANES and EXAFS data indicates that, only a small part of Cu(II) is inserted into the birnessite layers, while most of it is adsorbed on the vacancies. When the Cu/Mn molar ratio is increased from 0.08 to 0.23, an increasing part of Cu(II) is present as polynuclear clusters on the birnessite edge sites in the pH range of ~3.3-5.3. Reaction with H2SO4 solution is found to easily dissolve the polynuclear Cu clusters and the highly distorted Cu octahedra in innersphere complexes on the birnessite-water interface, with ~53% of the Cu2+ released into the solution. On the other hand, the reaction with HCl solution leads to reductive dissolution of the mineral matrix, the release of Mn2+ into solutions, the decrease in the first MnO and edge-sharing MnMn distances and Mn AOS, in addition to the release of Cu2+. The release rate of Cu2+ is much faster than that of Ni2+ in Ni-doped birnessites, owing to the lower stability of distorted [CuO6] octahedron upon proton attack. These results indicate the formation of multinuclear Cu complexes on birnessite surfaces under the investigated conditions. The results also suggest the lower stability of Cu2+ in these minerals and thus higher potential toxicity in acidic conditions, in comparison with other metal pollutants, such as Ni2+. This study provides new insights into the interaction mechanisms between Cu2+ and birnessite-like minerals, and help to clarify the structural stability and geochemical behaviors of Cu2+ associated with birnessite-like minerals in natural environments.